Techniques for Designing Low-Level Circuits

A special class of picoammeters and electrometers differentiate themselves from ordinary multimeters because they measure currents less than 100 attoA (10-16 A). They use very low offset-current components, and are made under a set of rigid design rules not widely known. However, the techniques that go into designing their low-level amplifiers can be used in many other kinds of electronics gear and should be part of all engineers’ knowledge base.

The main problem with designing circuits for handling low-level signals is eliminating the extraneous currents that corrupt them. Electrometer circuits nullify these currents and minimize the variables that generate them, which includes offset currents, leakage resistances, humidity, electrostatic interference, and triboelectric, piezoelectric, and electrochemical effects.

Offset currents seriously affect the accuracy of the input section of an electrometer or picoammeter, the stage most sensitive to errors. These currents may come from unbalanced input signals or high offset currents in integrated circuits. The first stage usually includes a dual input device, an operational amplifier, and a large feedback resistor that eliminates most offset current in conventional amplifiers. The amplifier output signal is normally a voltage, Vo = –IRf, where I = input current, A; and Rf = feedback resistor, Ω.

Most of the input current balances with current from the feedback resistor Rf, driving down the offset. Conventional IC operational amplifiers, however, can’t be used as input devices because their initial offset current is too high, even though it may only be a few picoamperes. But the offset current in a hybrid op amp designed with JFET or MOSFET transistors often falls below the maximum allowed. The hybrid benefits from the low-current offset of the FET input device, and the high gain and well-controlled frequency response of an integrated-circuit op amp.

MOSFETs are preferred because they have high input resistance, low noise current, and low input current. JFETs generally have lower input resistance, lower voltage noise and offset, but higher input current noise and offset. However, JFETs resist damage better from highvoltage overloads, so they need less-complex protection circuits.

After considering the trade-offs between the two devices, select one, and lay out the input circuit to ensure the highest possible resistance between the input and other PCB traces. This procedure usually takes care of excessive leakage currents in the input stage. However, the following guidelines are necessary to eliminate the remaining variables that also upset low-level signal integrity.

Signal-guarding techniques further lower leakage currents. A guard is a low-impedance point in the circuit that is at nearly the same potential as the high-impedance point. For example, surrounding the input terminals on the circuit board with a conductive trace guards the high impedance inputs of a packaged op amp. For inverting op amps, the guard trace connects to the noninverting input. For noninverting op amps, the guard connects to the output of unity-gain amplifiers.

Guarded circuits are often used in picoammeter input stages. For example, connecting a picoammeter in series with a diode to measure its reverse leakage current will also measure unwanted leakage currents through insulators and PC boards to ground. For insulation with a typical resistance of 1 GΩ and supplied by 15 Vdc, the current measured would be the sum of the leakage current of 15 nA and the diode reverse current.

But guarding the connection between the diode and the picoammeter so that it is completely surrounded by a conductor connected to the +15 Vdc is a viable solution. Because a typical feedback picoammeter has a maximum voltage burden of only 200 μV at full scale, this is the maximum voltage impressed upon RL. Thus, IL reduces by about 5 decades and is likely to be insignificant compared to ID. The current flowing through RG is still 15 nA, but this is supplied directly from the low-impedance +15-Vdc supply and thus is not measured by the ammeter.

Voltage sources also become a problem when they are not well insulated. They can generate leakage currents through high-resistance paths on circuit boards and interfere with the main signal. To help reduce this leakage, use high-quality insulators for standoffs and feed throughs. Also, use connectors with high-insulation resistance, such as Teflon, polyethylene, and sapphire.

Electrostatic-field interference crops up when an electrically charged object approaches an uncharged object. Usually, it does not interfere with low-level signals because the charge dissipates rapidly through low impedances. However, high-resistance materials do not discharge quickly and the charge can corrupt primary signals. To reduce the effects of these electrostatic fields, shield the circuits and all wiring. Shields should be conductive materials connected to the low-impedance point of the input circuit.

Frictional charges between certain materials such as insulators and conductors generate the Triboelectric currents when they rub together within flexing or vibrating coaxial cables. Free electrons leave the conductor, produce a charge imbalance, and generate current. But using low-noise cables impregnated with conductive lubricant greatly reduces friction and triboelectric effects. Also, make electrostatic shields rigid, and when possible, isolate sensitive circuits from all vibration sources.

Piezoelectric materials are a class of insulating materials that generate currents under mechanical stress and should be avoided in low-level amplifier circuits. These materials include quartz, various ceramics, and some plastics. A good rule of thumb is to reduce mechanical stress on all components and use insulators with a low piezoelectric effect, such as sapphire.

Electrochemical effects from leftover ionic chemicals on a circuit board create weak batteries between two conductors and generate interfering noise currents. For example, epoxy printed-circuit boards can generate up to a few nanoamperes if etching solution, flux, and other contaminants are not completely removed when the boards are cleaned. Other elements and materials also can generate noise currents and voltages such as connections made of dissimilar metals, dirty insulators, and excessive humidity. Choose insulating materials that do not readily absorb water vapor.

Thermal noise, the last factor, is found in all electronic equipment and is one of the toughest to reduce. Thermal energy produces charged particle motion in all resistive materials and generates electrical noise, sometimes called Johnson noise. Because most current sources contain an internal source resistance, they all exhibit this noise. Johnson noise is critical because it determines the smallest possible current that a circuit can detect.

The equation shows that decreasing the circuit resistance, the measurement bandwidth, or the source temperature reduces the noise current. Usually, circuit resistance cannot be reduced. The bandwidth might be reduced, but this increases the measurement time. For an extreme case, lowering the temperature of the source resistance from room temperature to the temperature of liquid nitrogen (77°K) reduces noise by about a factor of 2.